1. Field of the Invention
The present invention relates to a method of defect inspection of a graytone mask and a photomask including a microscopic pattern and an apparatus doing the same.
2. Description of the Related Art
In recent years, attempts have been made to reduce the number of mask sheets by using graytone masks in the field of large-sized LCD masks (as set forth in the monthly FPD Intelligence, May, 1999).
As shown in FIG. 6A, such a graytone mask has a opaque part 1, a transmission part 2 and a graytone part 3 on a transparent substrate. The graytone part 3 corresponds to an area in which a opaque pattern 3a of not exceeding the resolution limit of an exposure device for a large-sized LCD using the graytone mask is formed, for example, and is designed to selectively change the thickness of a photoresist film by decreasing the light transmitted through this area so as to decrease the amount of irradiation due to the area, 3b showing a microscopic transmission part of not exceeding the resolution limit of the exposure device in the graytone part 3. Normally, the opaque part 1 and the opaque pattern 3a are formed with films that are made of the same material such as chromium (Cr) or a chromium compound and have the same thickness. The transmission part 2 and the microscopic transmission part 3b are transparent substrate parts, each without having a opaque film on the transparent substrate.
The resolution limit of the exposure device for the large-sized LCD using the graytone mask is about 3 μm in the case of an exposure device of a stepper type and about 4 μm in the case of an exposure device of a mirror projection type. Consequently, the space width of a transmission part 3b in the graytone part 3 of FIG. 6A is set at less than 3 μm and the line width of the opaque pattern 3a of not exceeding the resolution limit of the exposure device is set at less than 3 μm, for example. When the exposure device for the large-sized LCD is used for light exposure, as the exposure light transmitted through the graytone part 3 as a whole is deficient in the amount of light exposure, positive photoresists are left on a substrate though the thickness of the positive photoresists exposed to light via the graytone part 3 solely decreases. More specifically, there arises a difference in solubility of resists in developing liquid between parts corresponding to the ordinary opaque part 1 and to the graytone part 3 because of difference in the amount of light exposure and this results in, as shown in FIG. 6B, making a part 1′ corresponding to the ordinary opaque part 1 as thick as about 1.3 μm, making a part 3′ corresponding to the graytone part 3 as thick as about 0.3 μm and making a part corresponding to the transmission part 2 a part 2′ without resists, for example. A first etching of a substrate as a workpiece is carried out in the part 2′ without the resists so as to remove the resists in the thin part 3′ corresponding to the graytone part 3 by ashing and the like and by carrying out a second etching of this part, the etching process is performed with one sheet of mask instead of two sheets of masks as conventionally used in order to cut down the number of masks for use.
A conventional method of inspection of a mask having only opaque and transmission parts will now be described.
FIG. 9A shows a condition in which a clear defect 11 (pinhole) and a opaque defect 12 (spot) are produced in a opaque part 1 and a transmission part 2 respectively with both parts being scanned by one of the lenses (hereinafter called an upper lens) of a comparative inspection device as shown by an arrow.
FIG. 9B shows an amount-of-transmission signal 13 obtainable along the scanning line of the lens. The amount-of-transmission signal 13 is detected by a CCD line sensor disposed in each lens unit, for example. The level of the amount-of-transmission signal 13 is B in the opaque part 1 and W in the transmission part 2. The transmittance of the opaque part 1 is set at 0% and the transmittance of the transmission part 2 is set at 100%. The amount-of-transmission signal 13 is basically formed of a pattern edge line signal (pattern form signal) generated at the edge (boundary between the opaque part and the transmission part) of the pattern. In case where defects are produced, there appear a clear defect signal 11′ generated in the opaque part 1 and a opaque defect signal 12′ generated in the transmission part 2.
FIG. 9C shows an amount-of-transmission signal 13′ obtainable by the other lens (hereinafter called a lower lens) in case where no defect is produced even in the same pattern as that of FIG. 9A.
FIG. 9D shows a difference signal 14 obtained by subtracting the amount-of-transmission signal (a different portion) obtained at each lens; more specifically, there is shown therein a difference signal obtained by subtracting the amount-of-transmission signal 13′ of FIG. 9C from the amount-of-transmission signal 13 of FIG. 9B. In the difference signal 14, only defect signals 11′ and 12′ are extracted because a pattern edge line signal is removed from the amount-of-transmission signal of each lens.
FIG. 9E shows a condition in which with the setting of thresholds necessary for extracting defects in the opaque part 1 and the transmission part 2 in the difference signal 14 that has extracted only defect signals, the clear defect is detected by a plus-side threshold 15a and the opaque defect is detected by a minus-side threshold 15b. Although the detection sensitivity increases as the thresholds lower, the thresholds are needed to be set at a level on which no false defects are picked up.
In order to make sure that what kind of defect is produced in which one of the lenses, the signal of the upper lens is compared with that of the lower lens in an upper lens circuit (by subtracting the signal of the lower lens from that of the upper lens), for example, so as to detect clear and opaque defects in the upper lens because a defect signal appears on the plus side when the clear defect is produced in the opaque part 1 of the upper lens and because a defect signal appears on the minus side when the opaque defect is produced in the transmission part 2 of the upper lens (FIG. 9B-(5)). In the same way, the signal of the lower lens is compared with that of the upper lens in a lower lens circuit (by subtracting the signal of the upper lens from that of the lower lens), for example, so as to detect clear and opaque defects in the lower lens because a defect signal appears on the plus side when the clear defect is produced in the opaque part 1 of the lower lens and because a defect signal appears on the minus side when the opaque defect is produced in the transmission part 2 of the lower lens.
As the conventional comparative inspection device mentioned above is a device for inspecting a conventional mask only having a opaque and a transmission part, it is unfit for inspecting a graytone mask having a graytone part.
More specifically, the following problems develop in case where the conventional comparative inspection device is used for inspecting a graytone mask.
As the defect signal in the graytone part is weak since the defect itself is very small and when the conventional comparative inspection device is employed, it is difficult to make defect inspection unless the threshold is set lower than a threshold normally used to inspect the opaque part. However, in case where the graytone part is an area wherein a microscopic pattern of not exceeding the resolution limit of the exposure device using the graytone mask, for example, a base signal level 16 (noise band) characteristic of the graytone part as shown in FIG. 5 is generated as what corresponds to the microscopic pattern. In making the comparative inspection, though only a defect signal is extracted by obtaining a difference signal after subtracting the amount-of-transmission signal (a different portion) obtained at each lens, the base signal level is amplified (by maximum two folds) when as light pattern shift is generated between the microscopic opaque patterns in the graytone part and a defect (false defect) that should not originally be treated as a defect comes to be detected; consequently, the threshold becomes impossible to lower and the problem is that high-sensitivity inspection cannot be fulfilled.
Further, as the comparative inspection conventionally carried out is intended to inspect clear and opaque defects, it remains difficult to ensure a transmittance as the most important factor in the graytone mask. In other words, in case where the line width of a opaque pattern in a graytone part is too large or too small as compared with a designed dimension, thus making the transmittance exceed the allowed value over the whole mask area and in case where the transmittance of a translucent film forming the graytone part exceeds the allowed value, for example, as the difference signal obtained by subtracting the amount-of-transmission signal of each lens in the comparative inspection, there is also a problem arising from making the transmittance undetectable because the difference will not appear. This is particularly problematical in that no form defect exists in the graytone part. Further, though the transmittance of the graytone part need not be detected as a defect as long as the transmittance thereof remains in the allowable range, since even the transmittance thereof within that allowable range is still detected in the conventional comparative inspection, there may be a case where the transmittance thereof thus detected remains in the allowable range. As a result, there is still a problem developed from failing to maintain inspection accuracy (capability) as what has originally been unnecessary to detect as a defect is subjected to detection.
There is additionally caused a similar problem to a photomask having a microscopic pattern such as a photomask for forming a TFT channel, for example. In the case of such a photomask for forming the TFT channel, for example, with the progress of rendering the TFT channel portion microscopic, the tendency is for the pattern to be rapidly made microscopic. In the case of even a pattern like this, there occur a false defect due to the vibration of the stage of the inspection device and a shift between the upper and lower lenses and any other false defect characteristic of the microscopic pattern when the inspection is carried out through the conventional method; the problem in this case is that a sensitivity at a level ensuring the defect detection is not secured when the sensitivity is lowered up to the level at which the false defect is not detected.